1. Field of the Invention
The present invention relates to managing access to assets in distributed data storage systems such as file systems and databases.
2. Description of the Related Art
Distributed file systems are used to provide data sharing in distributed computer systems. Such systems centralize data storage, which improves the scalability and manageability of data access control. Moreover, centralized data storage also facilitates, among other things, easier storage device replacement and data backups, as compared to systems in which data storage is fragmented among local storage devices of many computers. It is to be understood that while, for disclosure purposes, the present discussion focuses on file systems, the principles set forth herein apply equally to other distributed data storage systems, such as distributed database systems.
To synchronize data access such that users share consistent views of shared data, requests from users to read and write data typically are sent to a central file server. The file server then manages access to the data using xe2x80x9clocksxe2x80x9d to ensure, e.g., that one user is not updating shared data by writing to it while another user might read an out-of-date version of the same data. In distributed systems, locks ordinarily are preemptible, in that the server can demand a lock previously provided by the server to one user to enable the user to access a file, and then give the lock to a second user. Unfortunately, requiring a central server to actively manage all data access degrades performance as compared to accessing a local data cache, since the server essentially represents a bottleneck.
As recognized by the present invention, while a central server can be used to manage synchronized access and coherent views of data, to optimize system performance the server should not be used as a target for all read and write requests. Stated differently, the present invention recognizes that it is desirable to provide local storage system semantics in a distributed environment, wherein communications with a server is minimized where possible. In this way, the speed, ease, and efficiency of accessing assets in a distributed storage network can approach that of accessing data in a local cache. Furthermore, the present invention understands that any asset locking scheme preferably be amenable to simplification, to further improve system performance.
A general purpose computer is programmed according to the inventive steps herein to manage access to assets in a distributed storage system. The invention can also be embodied as an article of manufacturexe2x80x94a machine componentxe2x80x94that is used by a digital processing apparatus and which tangibly embodies a program of instructions that are executable by the digital processing apparatus to execute the present logic. This invention is realized in a critical machine component that causes a digital processing apparatus to perform the inventive method steps herein.
The invention can be implemented by a computer system including at least one general purpose client computer, at least one general purpose server computer, and a distributed data storage system accessible to at least the client computer. The system also includes logic that can be executed by the client computer for undertaking method acts to manage access to assets in the storage system. The method acts undertaken by the client computer include sending a request for a first semi-preemptible access lock from the client computer to the server computer. The access lock can be thought of as a distributed lock that encapsulates local open instances protected by non-preemptible local locks, also referred to as file locks.
Also, the method acts include receiving the first access lock from the server computer, it being understood that the access lock pertains to at least one asset in the storage system. The asset is characterized by either an open state or a closed state. A demand can be subsequently received from the server computer for the first access lock, and the method includes selectively not relinquishing the first access lock if the open state exists for the asset, and otherwise relinquishing the first access lock.
The preferred method undertaken by the client computer includes, if an open state exists for the asset, attempting to downgrade incompatible locks held by the client computer, with the incompatible locks being characterized as being incompatible with the first access lock. The first access lock is not relinquished if any incompatible lock cannot be downgraded.
In another aspect, a computer system includes at least one general purpose client computer, at least one general purpose server computer, and a distributed data storage system accessible to at least the client computer. The system also includes logic that can be executed by the server computer for undertaking method acts to manage access to assets in the storage system. The method acts undertaken by the server computer include receiving a request for a first semi-preemptible access lock from a first client computer, and determining at least whether the first lock is compatible with a second semi-preemptible lock associated with a second client computer. Also, the logic includes granting the request if the first lock is compatible with the second lock, and otherwise demanding the second lock.
In still another aspect, a computer-implemented method for managing access among plural client computers to assets in a distributed data storage system associated with at least one server computer includes issuing semi-preemptible access locks to client computers. In accordance with present principles, the semi-preemptible access locks are conditions precedent for the grant of a file lock to open a file. The semi-preemptible access locks are relinquished upon demand of the server computer when no associated file lock is invoked.
The details of the present invention, both as to its structure and operation, can best be understood in reference to the accompanying drawings, in which like reference numerals refer to like parts, and in which: